In LCD, linear polarizing plates or circular polarizing plates are used to control optical rotation or birefringence in display. Also in OLED, circular polarizing plates are used to prevent reflection of outside light. Heretofore, for such polarizing plates (polarizing elements), iodine has been widely used as a dichroic material. However, if iodine is used for a polarizing film, its heat resistance or light fastness is inadequate since iodine is highly sublimable. Further, the extinction color becomes dark grayish blue, and an ideal achromatic color polarizing plate for the entire visible spectral region cannot necessarily be obtained.
Therefore, an anisotropic dye film as a polarizing film has been studied wherein an organic dye is used as a dichroic material which replaces iodine. However, such an organic dye has a problem such that only polarizing elements are obtainable which are substantially inferior in dichroism as compared with ones employing iodine.
Particularly, a polarizing element is an important constituent in LCD employing as the display principle optical rotation or birefringence of light, and a new polarizing element has been developed for the purpose of improving display performance and the like in recent years.
As one method of forming an anisotropic dye film to be used for such a polarizing element, a method may be mentioned wherein, in the same manner as in the case of a polarizing film containing iodine, an organic dye having dichroism (dichroic dye) is resolved or adsorbed in a polymer material such as a polyvinyl alcohol, and the obtained film is stretched in one direction into a film so that the dichroic dye is oriented.
Further, Non-Patent Document 1 discloses a method of orientating a dichroic dye on a substrate such as glass or a transparent film utilizing e.g. intermolecular interaction of organic dye molecules.
The method of orienting a dichroic dye on a substrate such as glass or a transparent film utilizing e.g. intermolecular interaction of organic dye molecules to prepare an anisotropic dye film, may be a wet system film-forming method. In a case where an anisotropic dye film is prepared by the wet system film-forming method, the dye molecules to be used for the dye film are required not only to show high dichroism but also to be a dye suitable for the process for the wet system film-forming method.
Further, Patent Documents 1 to 3 propose materials suitable for the process of the wet system film-forming method. However, although such materials are suitable is for the process, they have had such a drawback that they cannot show high dichroism.
Non-Patent Document 1: Dreyer, J. F., Phys. And Colloid Chem. 1948, 52, 808., “The Fixing of Molecular Orientation”
Patent Document 1: JP-A-2002-180052
Patent Document 2: JP-A-2002-528758
Patent Document 3: JP-A-3-78703